annex.skipunknown with transition plan

Added annex.skipunknown git config, that can be set to false to change the
behavior of commands like `git annex get foo*`, to not skip over files/dirs
that are not checked into git and are explicitly listed in the command
line.

Significant complexity was needed to handle git-annex add, which uses some
git ls-files calls, but needs to not use --error-unmatch because of course
the files are not known to git.

annex.skipunknown is planned to change to default to false in a
git-annex release in early 2022. There's a todo for that.

Command/PreCommit.hs

@@ -32,7 +32,8 @@ cmd = command "pre-commit" SectionPlumbing
 
 seek :: CmdParams -> CommandSeek
 seek ps = do
-	l <- workTreeItems ps
+	let ww = WarnUnmatchWorkTreeItems
+	l <- workTreeItems ww ps
 	-- fix symlinks to files being committed
 	flip withFilesToBeCommitted l $ \f -> commandAction $
 		maybe stop (Command.Fix.start Command.Fix.FixSymlinks f)